Sustained release solid dosage forms are conventionally produced by coating active ingredients with a water-insoluble polymer and formulating the coated active ingredients into final dosage forms together with various excipients. As such the conventional sustained release formulations rely on a mechanism in which dissolution of active ingredients in the digestive juice is controlled by the coating film of water-insoluble polymers.
U.S. Pat. No. 5,486,364 to King et al. discloses sustained release pharmaceutical tablets based on another mechanism. The sustained release tablets are produced by direct compression of dry powders of readily available konjac glucomannan alone or mixtures thereof with xanthan gum admixed with active ingredients. Examples of the readily available konjac glucomannan include clarified konjac glumomannan, cryogenetically ground konjac glucomannan and plasticized konjac glucomannan.
The dissolution properties of sustained release tablets produced in Examples are given in the specification in terms of cumulative percentage of theophiline which was released into deionized water (pH=7) during 6 hour elapsed time. The cumulative percentage varies from 14% to 74% depending upon the particular type of konjac glucomannan.
It is known that most of drugs are absorbed from the small intestine when administered orally and that the sum of the gastric emptying time and the small intestine transit time of tablets in adult human is about 6 hours or less. Because of this, a portion of the active ingredient which was not released from the solid dosage forms in the digestive tract up to the end of small intestine transit time will be excreted to outside the body and will not be available for the intended therapeutic purposes. A need exists, thereof, for a sustained release solid dosage preparation having enhanced bioavailability.